When communication is performed on a network, in particular, a network used in an FA (Factory Automation) system and required to have real-time properties, it is desired that a communication delay is within a predetermined time and, at the same time, there is no loss of information.
In general, for measurement of a delay, there are a method of measuring a delay time in a round trip between two nodes to be measured and a method of measuring a delay time in one way between the nodes. In the measurement of a delay time in one way, a delay can be determined at a point when a reception side receives a communication frame. Therefore, compared with the method of measuring the round-trip delay time, there is an advantage that a time required for the measurement of a delay can be reduced. On the other hand, to perform the measurement of a delay time in one way, clocks need to be synchronized between the nodes or an offset time of clocks needs to be calculated between the nodes.
The measurement of a delay time in one way is performed in Patent Literature 1 as explained below. First, an offset time of clocks is calculated. Subsequently, a node on a transmission side gives a time stamp of a transmission time to a packet to be transmitted and transmits the packet. Thereafter, a node on a reception side records a time stamp of a reception time of the packet. The node on the reception side calculates a delay using the offset time of the clocks between the nodes, the time stamp of the transmission time, and the time stamp of the reception time.
The calculation of an offset time of clocks is performed as explained below. It is assumed that the nodes have a time calculating function. First, a first node transmits, to a second node, a packet for shift time calculation to which a time stamp of a transmission time acquired from a clock of the first node is given. Subsequently, the second node adds, to the received packet, a packet reception time from the first node and a transmission time in returning a packet to the first node and returns the packet to the first node. The first node records a reception time of the returned packet and calculates a shift time based on these four times.
On the other hand, the clock synchronization between the two nodes is performed in Patent Literature 2 as explained below. First, a first node creates a measurement packet in which a transmission time is put in a first payload and transmits the measurement packet to a second node. Subsequently, when the second node receives the measurement packet from the first node, the second node creates a return packet in which a transmission time of the measurement packet is put in a first payload, a reception time of the measurement packet is put in a second payload, and a transmission time of the return packet is put in a third payload and transmits the return packet to the first node. The first node that receives the return packet records a reception time of the return packet and performs correction of a clock based on these four times.
Concerning a loss of information (a packet), for example, in Patent Literature 1, a packet loss ratio calculating function is provided in a node, a sequence number is given to a transmission packet, and the number of losses of packets is counted according to missing of the sequence number.